how to set camera's look-at point when moves along a curve?
You can access and alter the properties of the constraints of an object, all constraints are listed in object.constraints, access each constraint by its name or type and of course the index

cam = scene.camera
# if you are sure the constraint exist and has the default name
fol_path = cam.constraints['Follow Path']

fol_path.forward_axis = 'TRACK_NEGATIVE_Z'
fol_path.up_axis = 'UP_Y'

# to avoid errors in case it doesn't exist or has another name
for cons in cam.constraints :
    if cons.type == 'FOLLOW_PATH':
        cons.forward_axis = 'TRACK_NEGATIVE_Z'
        cons.up_axis = 'UP_Y'